Hyperactive Innate Immunity Causes Degeneration of Dopamine Neurons upon Altering Activity of Cdk5

Abstract

Innate immunity is central to the pathophysiology of neurodegenerative disorders, but it remains unclear why immunity is altered in the disease state and whether changes in immunity are a cause or a consequence of neuronal dysfunction. Here, we identify a molecular pathway that links innate immunity to age-dependent loss of dopaminergic neurons in Drosophila. We find, first, that altering the expression of the activating subunit of the Cdk5 protein kinase (Cdk5α) causes severe disruption of autophagy. Second, this disruption of autophagy is both necessary and sufficient to cause the hyperactivation of innate immunity, particularly expression of anti-microbial peptides. Finally, it is the upregulation of immunity that induces the age-dependent death of dopaminergic neurons. Given the dysregulation of Cdk5 and innate immunity in human neurodegeneration and the conserved role of the kinase in the regulation of autophagy, this sequence is likely to have direct application to the chain of events in human neurodegenerative disease.

Keywords: Cdk5; Drosophila; aging; antimicrobial peptides; autophagy; innate immunity; neurodegeneration; p35.

Figure 1.. Both Gain and Loss of Cdk5α Induce the Degeneration of Dopaminergic Neurons

Brains of 3-, 10-, 30-, and 45-day-old flies of the indicated genotypes were dissected and immunostained with anti-DTH antibody. (A) Representative projected confocal images of DA neurons labeled with anti-DTH antibody. (B) The number of DTH⁺ DA neurons per hemisphere is presented as mean ± SEM, along with individual counts. The pooled DA neuron count includes neurons from PPL1, PPL2, PPM1/2, PPM3, and PAL clusters. For individual counts in these clusters, see Figure S1. For each genotype and age, the number of hemispheres examined is presented at the bottom of the bar. Error bars indicate SEM. The significance of differences is relative to the age-matched control (one-way ANOVA with Dunnett’s multiple correction). The DA neuron count in 45-day-old Cdk5α-OE shows an apparent rebound in numbers relative to 30 days; this reflects selective survival of only the fittest individuals at the oldest time point (Spurrier et al., 2018). For complete details of statistical analysis for this and all of the figures, see Table S2. (C) Top: projected confocal images of 30-day-old brains of DTH-stained control, Cdk5α null, and Rescue {Cdk5α null; Tn[Cdk5α]/+}. Bottom: counts showing mean ± SEM along with individual values. For the rescue samples, the significance of differences was calculated between rescue and age-matched Cdk5α null using one-way ANOVA with Tukey’s multiple correction. In the rescue calculation, the same data for 30-day-old controls and Cdk5α null were used.

Figure 2.. Altered Expression of Cdk5α Causes Overexpression of Anti-microbial Peptides

(A) Principal-component analysis (PCA) of micro-array expression profiling data. Colored ovals represent individual samples (N = 5 biological replicates), color coded as indicated. PC2 is found to separate samples essentially by effective age; PC3 separates by the severity of the neurodegenerative phenotype. (B) Z scores of genes that make a statistically significant contribution to PC3. AMPs and other secreted innate immune proteins are represented by orange. Genes with predicted function as proteases and hydrolases are shown in blue. (C) qRT-PCR for the expression level of the AMPs listed (color key at top). RNA was isolated from the heads of flies of the indicated genotypes and ages. Fold change of AMP expression was determined relative to 3-day-old controls, with rp49 as an endogenous control, and is displayed as mean ± SEM for three biological replicates. Student’s t test, as compared to 3-day-old control, was used to calculate significance (Table S2). (D) qRT-PCR for AMPs was performed and quantified as above, using RNA from the heads of 30-day-old flies that were Cdk5α null, without (−) or with (+) Tn[Cdk5α⁺] rescuing genomic transgenes. Statistical significance was assessed by t test for each AMP. Note that this transgene expresses at a lower level than the endogenous locus, so that the rescue of the phenotype is only expected to be partial (Spurrier et al., 2018). Student’s t test, as compared to 30-day-old Cdk5α null was used to calculate significance. (Three biological replicates; error bars indicate SEM).

Figure 3.. Cdk5α-Altered Drosophila Have Neuronal Expression of AMPs

(A) Projected confocal image showing drosomycin expression (green) in the brains of 30-day-old Cdk5α null Drosophila along with embryonic lethal, abnormal vision (ELAV) (red, neuronal marker) and reversed polarity (Repo) (magenta, glial marker). Yellow arrowheads highlight drosomycin in glial cells, while white arrows highlight expression in neurons. Some GFP⁺ cells were observed that were positive for neither neuronal nor glial markers (stars); some of these resemble hemocytes. Five biological replicates were examined. (B) AMP expression in Cdk5α null flies with or without Elav-GAL4-mediated knock down of Relish expression. Fold change of AMPs was calculated versus 30-day-old controls and presented as mean ± SEM; significance in Elav-GAL4; Cdk5α null was assessed for each AMP by comparison to age-matched Elav-Gal4, while for Elav-Gal4;Cdk5α null; Relish RI, it was compared to Elav-Gal4;Cdk5α null, using Student’s t test for five biological replicates. (C) DA neuron counts in 30-day-old Elav-Gal4, Elav-Gal4;Cdk5α null, and Elav-Gal4;Cdk5α null;Relish RI. Flies were aged at 25°C, and DA neurons were counted by staining with anti-TH antibody. Data are presented as mean ± SEM with individual counts shown. The number of brain hemispheres counted are at the bottom of each bar. Statistical significance was determined using one-way ANOVA with Tukey’s multiple correction.

Figure 4.. Overexpression of AMPs Induces Loss of DA Neurons

(A) TH-Gal4 was used to drive the indicated AMP genes in DA neurons, and the neuron number was counted at 3 and 30 days by immunostaining with anti-TH antibody. The graphs depict mean ± SEM for the four experimental conditions, and mean ± SD for the comparison of 3-day-old versus 30-day-old control; individual counts also are shown. The number of brain hemispheres counted is at the bottom of each bar. UAS-Drosocin and UAS-Metchnikowin flies show significant GAL4-independent expression and GAL4-independent loss of DA neurons at 30 days of age. For the expression level of these AMPs, see Figure S3. Significance analysis for TH-Gal4 was done using an unpaired t test, while significance was measured by one-way ANOVA with Tukey’s multiple correction for others. (B) qRT-PCR for the AMPs shown was performed on the RNA from the heads of 30-day-old flies of the indicated genotypes. Fold change of AMPs was calculated relative to 3-day-old controls. Statistical significance for Cdk5α null was assessed as compared to 30-day-old controls, while significance for Cdk5α null;Rel^E20/TM6B was compared to the fold change in Cdk5α null. Statistical significance was assessed by Student’s t test (three biological replicates; error bars indicate SEM). (C) Males of the indicated genotypes were aged at 25°C, and the brains were fixed, dissected, and examined for DA neuron number by immunostaining with anti-TH antibody at 30 days of age. Data are presented as mean ± SEMs, with individual values shown. The number of brain hemispheres examined is presented at the bottom of each bar. Statistical significance was determined using one-way ANOVA with Tukey’s multiple correction.

Figure 5.. Altered Cdk5α Levels Reduce Autophagy Efficiency

(A and B) Flies of the indicated genotypes were aged to 30 days, and brains were fixed, dissected, and labeled with DAPI (blue), anti-TH (green), and anti-Ref(2)P (red). (A) Projected confocal images, including PPL1 cluster, showing separated channels and merged image, as well as a 3D surface rendering of a single TH⁺ cell (dotted yellow rectangle in merged image). (B) Ref(2)P⁺ puncta were counted using the ImageJ (NIH) particle counting tool, and data are presented as mean ± SEM, with individual values shown. Brain region including PPL1 cluster was examined for six hemispheres per genotype, and puncta were counted in two size ranges, 0.02–0.40 μm² and >0.40 μm². Statistical significance was assessed by one-way ANOVA with Dunnett’s multiple correction. For a 3D view documenting the localization of Ref(2)P puncta inside the DA neuron, the 3D cropping tool of Imaris (Bitplane) was used, followed by surface rendering and manual pruning of puncta outside the DA neuron. (C) TH-Gal4 was used to drive UAS-GFP-mCherry-Atg8a in DA neurons. Brains were dissected without fixation, and fluorescence was examined. DA neurons of control flies (w⁺;UAS-GFP-mCherry-Atg8a/CyO;TH-Gal4/TM6B) have mostly mCherry⁺ puncta, while Cdk5α null flies (w⁺; Cdk5α/DfC2,UAS-GFP-mCherry-Atg8a;TH-Gal4/TM6B) have mostly double-positive puncta (yellow). Six biological replicates were used for this experiment. (D) Western immunoblot with anti-GFP antibody was used to quantify Vha13 protein in the extract of 30-day-old heads of flies of the indicated genotypes bearing a Vha-13-GFP gene trap. Left: typical immunoblot; numbers give the molecular weights of the markers. Right: quantification, displaying mean ± SEM (average of three biological replicates, normalized with anti-tubulin as loading control). Significance determined by paired t test. (E) qRT-PCR was performed in biological triplicate, as above, to quantify Mitf expression in the RNA of the heads of the indicated genotypes. Error bars indicate SEM. UAS-Mitf was driven with a third chromosome insert of ELAV-GAL4 that has very low adult expression (BL8760). (F) Western immunoblot (left) and quantification (right) of cysteine protease (Cp1), detected with anti-Cp1 antibody in the extract of the heads of the indicated genotypes isolated at 30 days of age. The Cp1 value presented is an average of three biological replicates, normalized with anti-tubulin as the loading control. Error bars indicate SEM. Raw intensity values from all of the trials are given in Table S4. Significance was assessed by one-way ANOVA with Dunnett’s multiple correction. The molecular weights of the markers are indicated next to the blots.

Figure 6.. Reduced Autophagy Induces AMP Expression and DA Neuron Loss in Aged Flies

(A) Ref(2)P accumulation in the brains of Atg8a¹/Y flies. Flies were aged to 30 days; brains were fixed, dissected, and stained with DAPI (blue), anti-TH (green), and anti-Ref(2)P (red). Separated channels and merged image are shown as projected confocal images; the rightmost panel is a 3D rendering of a single TH⁺ cell showing Ref(2)P accumulation inside the cell (cell highlighted with yellow box in the merged image). Five biological replicates were examined for Ref(2)P accumulation. (B) AMP expression in the heads of Atg8a¹/Y flies. RNA was isolated from the heads of 30-day-old flies that were Atg8a¹/Y or controls, and qRT-PCR was performed, as above. Bars show fold change of the expression level in mutants relative to age-matched controls (mean ± SEM; significance was calculated using Student’s t test; five biological replicates). (C) DA neuron counts in the brains of Atg8a¹/Y flies and controls. Flies were aged to 30 days, and the brains were fixed, dissected, and assayed for DA neuron number by fluorescence microscopy after immunostaining with anti-TH. Bars indicate mean ± SEM with individual values shown. The number of brain hemispheres examined is at the bottom of each bar, and significance was assessed relative to controls (t test). (D) Ref(2)P accumulation in the brains of controls and 30-day-old Cdk5α null without and with low-level, GAL4-driven expression of Mitf. Top: projections of the brains of the indicated genotypes, which were fixed, dissected, and immunostained with anti-Ref(2)P. Bottom: quantification of Ref(2)P puncta from six to eight brain hemispheres of each condition. Data are presented as mean ± SEM, along with individual counts. Two-way ANOVA with Tukey’s multiple correction was used for statistical analysis. ELAV-Gal4 > UAS-Mitf expression generated general cytoplasmic background label with this antibody; background subtraction was performed by rolling circle (5.0 pixels) before counting the puncta. For an unprocessed image, see Figure S6. (E) AMP expression levels in Cdk5α null flies without and with ELAV-driven restoration of Mitf. qRT-PCR for the AMP level was performed on the RNA from the heads of 30-day-old flies, as previously described. Fold change of AMPs was calculated versus 30-day-old controls and presented as mean ± SEM; significance was assessed for each AMP by comparison to age-matched Cdk5α null using three biological replicates and Student’s t test. (F) DA neuron counts in 30-day-old Cdk5α null and Cdk5α null; ELAV-Gal4/UAS-Mitf. Flies were aged at 25°C, and DA neurons were counted, as before, by staining with anti-TH antibody. Data are presented as mean ± SD, with individual counts shown. The number of brain hemispheres counted are at the bottom of each bar, and significance is by comparison to Cdk5α null (Student’s t test).